Particulate detection system

ABSTRACT

A particulate detection system ( 1 ) for detecting the quantity of particulates S in a gas under measurement EG, including a detection section ( 10 ), a drive circuit ( 210, 240 ), and a control section ( 230, 202 ). The detection section ( 10 ) has an ion source ( 11 ) and a particulate electrification section ( 12 ). The drive circuit ( 210 ) includes an ion source drive circuit ( 210 ) for performing constant current control. The control section ( 230, 202 ) includes current convergence determination means S 2 -S 3 , S 5 -S 6 , and detection start means S 8  for starting detection of the quantity of the particulates S using the signal Is, detected by a detection circuit ( 230 ), after the gaseous discharge current Id has converged to an allowable range IR.

TECHNICAL FIELD

The present invention relates to a particulate detection system fordetecting the quantity of particulates contained in a gas undermeasurement which flows through a gas flow pipe.

BACKGROUND ART

Exhaust gas discharged from an internal combustion engine (for example,a diesel engine or a gasoline engine) may contain particulates such assoot.

Exhaust gas containing such particulates is purified by means ofcollecting the particulates through use of a filter. Therefore, if thefilter suffers breakage or a like failure, unpurified exhaust gas isdischarged directly to the downstream side of the filter.

Therefore, there has been demanded a particulate detection system whichcan detect the quantity of particulates contained in exhaust gas inorder to directly measure the quantity of particulates contained inexhaust gas or to detect a failure of the filter.

For example, Patent Document 1 discloses a particulate measurementmethod and apparatus. In the method disclosed in Patent Document 1,clean gas is ionized by means of corona discharge, the ionized gascontaining positive ions is mixed with exhaust gas which is introducedfrom an exhaust pipe into a channel and which contains particulates, soas to electrify the particulates, and the particulates are then releasedto the exhaust pipe. A current (signal current) which flows inaccordance with the quantity of the released, electrified particulatesis detected so as to detect the particulate concentration.

Also, Patent Document 2 discloses a concrete structure of a particulatesensor which uses such ions generated by means of corona discharge. Asdisclosed in Patent Document 2, a constant current circuit is generallyused as a power supply circuit for supplying electrical power for coronadischarge, and when corona discharge is to be produced, a constantcurrent of, for example, about 5 μA is supplied to an electrode forcorona discharge.

PRIOR ART DOCUMENTS Patent Documents

-   Patent Document 1: Japanese Kohyo (PCT) Patent Publication No.    2011-513742-   Patent Document 2: Japanese Patent Application Laid-Open (kokai) No.    2012-194077

SUMMARY OF THE INVENTION Problems to be Solved by the Invention

Incidentally, in order to accurately detect the quantity ofparticulates, the current (e.g., 5 μA) controlled by the above-mentionedconstant current circuit must be stable at a fixed level. However, inmany cases, the current supplied for corona discharge is unstableimmediately after the start of corona discharge. In such a state, thequantity of ions generated by means of corona discharge varies.Therefore, if the quantity of particulates is detected in the state inwhich the current is unstable, difficulty is encountered in accuratelydetecting the quantity of particulates.

The present invention has been accomplished in view of such a problem,and its object is to provide a particulate detection system which canaccurately detect the quantity of particulates by preventing thedetection from being performed in a state in which detection accuracy islow.

Means for Solving the Problems

One mode of the present invention is a particulate detection system fordetecting the quantity of particulates contained in a gas undermeasurement flowing through a gas flow pipe, comprising a detectionsection attached to the gas flow pipe; a drive circuit for driving thedetection section; and a control section for controlling the drivecircuit and detecting the quantity of the particulates. The detectionsection includes an ion source for generating ions by means of gaseousdischarge, and a particulate electrification section for mixing aportion of the gas under measurement with the ions to thereby produceelectrified particulates which originate from the particulates withinthe gas under measurement and which carry the ions adhering thereto. Thedrive circuit includes an ion source drive circuit for performingconstant current control such that the gaseous discharge currentsupplied to the ion source becomes equal to a predetermined targetcurrent. The control section includes a detection circuit for detectinga signal corresponding to the quantity of the electrified particulates,current convergence determination means for determining whether or notthe gaseous discharge current supplied from the ion source drive circuithas converged to a predetermined allowable range including the targetcurrent after operation of the ion source by the ion source drivecircuit had been started, and detection start means for startingdetection of the quantity of the particulates through use of the signalafter the gaseous discharge current has converged to the allowablerange.

In this particulate detection system, the ion source drive circuitperforms constant current control such that the gaseous dischargecurrent supplied to the ion source becomes equal to the predeterminedtarget current.

However, as described above, the gaseous discharge current is unstableimmediately after the operation of the ion source by the ion sourcedrive circuit has been started. Also, when the insulation properties ofthe ion source has deteriorated due to adhesion of condensed water orsoot to the circumference of the ion source, the gaseous dischargecurrent may take a great deal of time to converge to the target current.In such a state where the gaseous discharge current is unstable, thequantity of the generated ions varies. Therefore, difficulty isencountered in accurately detecting the quantity of particulates throughuse of a signal detected by the detection circuit and corresponding tothe quantity of electrified particulates (for example, a current whichflows in accordance with the quantity of electrified particulates).

In view of the foregoing problem, in this particulate detection system,the detection of the quantity of particulates through use of the signalis started after the gaseous discharge current supplied to the ionsource has converged to a predetermined allowable range after theoperation of the ion source had been started.

By virtue of this, the detection of the quantity of particulates throughuse of the signal detected by the detection circuit can be started in astate in which the gaseous discharge current is stable. Therefore, thequantity of generated ions becomes stable, and the quantity ofparticulates can be detected accurately.

Notably, as described above, an example of the signal detected by thedetection circuit and corresponding to the quantity of electrifiedparticulates is a current corresponding to the quantity of electrifiedparticulates. Also, an example of a method of detecting the quantity ofelectrified particulates through use of the signal detected by thedetection circuit is a method of converting the detected signal(current) to the quantity of electrified particulates by using apredetermined conversion equation or a predetermined reference table.Also, the magnitude of the current detected by the detection circuit maybe used as a physical quantity corresponding to the quantity ofelectrified particulates, without performing such conversion.

Further, in the above-described particulate detection system,preferably, the detection section includes a heater for heating the ionsource; the drive circuit includes a heater energization circuit forenergizing the heater; and the control section includes heaterenergization control means for causing the heater energization circuitto energize the heater until the gaseous discharge current converges tothe allowable range.

In this particulate detection system, foreign substances, such as waterdroplets (e.g., condensed water) and soot, which adhere to thecircumference of the ion source can be removed by the heater, wherebythe insulation properties of the ion source can be recovered. Thus, thegaseous discharge current can be quickly converged to the allowablerange, whereby the time required to start the detection of the quantityof particulates can be shortened.

Further, in the above-described particulate detection system,preferably, the heater energization control means includes heaterenergization start means for causing the heater energization circuit tostart the energization of the heater when the gaseous discharge currentdoes not converge to the allowable range within a predetermined periodof time after the operation of the ion source has been started.

In this particulate detection system, when the gaseous discharge currentdoes not converge to the allowable range within a predetermined periodof time after the operation of the ion source has been started, theheater energization circuit is caused to start the energization of theheater.

Accordingly, in this particulate detection system, the heater is notenergized in the case where, at the start of the operation of the ionsource, the gaseous discharge current converges to the allowable rangeand the detection of the quantity of particulate can be started.Therefore, the electrical power consumed as a result of the energizationof the heater can be reduced.

Further, in any of the above-described particulate detection systems,preferably, the detection section includes a collection electrode forcollecting floating ions which are a portion of the ions and whichfailed to adhere to the particulates when the ions was mixed with thegas under measurement, and an auxiliary electrode for assisting thecollection of the floating ions by the collection electrode; the drivecircuit includes an auxiliary electrode drive circuit for driving theauxiliary electrode; and the heater is configured to heat the auxiliaryelectrode in addition to the ion source.

In this particulate detection system, an auxiliary electrode isprovided, and the heater heats the auxiliary electrode in addition tothe ion source. By virtue of this configuration, it is possible torecover the insulation properties of the auxiliary electrode havingdeteriorated as a result of adhesion of foreign substances, such ascondensed water and soot, to the auxiliary electrode. Therefore, thequantity of particulates can be detected more properly.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 Explanatory view showing a particulate detection system accordingto an embodiment which is applied to an exhaust pipe of an enginemounted on a vehicle.

FIG. 2 Explanatory view schematically showing the configuration of theparticulate detection system according to the embodiment.

FIG. 3 Explanatory view schematically showing introduction ofparticulates into a particulate electrification section of theparticulate detection system according to the embodiment,electrification of the particulates, and release of the electrifiedparticulates from the particulate electrification section.

FIG. 4 Explanatory view of a portion of the particulate detection systemaccording to the embodiment; i.e., an auxiliary electrode member and anauxiliary electrode insulating pipe with a heater which covers theauxiliary electrode member.

FIG. 5 Graph relating to the particulate detection system according tothe embodiment and showing a change in discharge current with time afteroperation of an ion source is started.

FIG. 6 Flowchart showing operation of the particulate detection systemaccording to the embodiment.

FIG. 7 Flowchart showing operation of an initial convergencedetermination sub-routine according to the embodiment.

FIG. 8 Flowchart showing operation of a convergence determinationsub-routine according to the embodiment.

MODE FOR CARRYING OUT THE INVENTION

A particulate detection system 1 according to the present embodimentwill be described with reference to the drawings. The particulatedetection system 1 of the present embodiment is attached to an exhaustpipe EP of an engine ENG (internal combustion engine) mounted on avehicle AM, and detects the quantity of particulates S (soot, etc.)contained in exhaust gas EG flowing through the exhaust pipe EP (seeFIG. 1). This system 1 is mainly composed of a detection section 10, acircuit section 201, and a feed pump 300 which is a compressed airsource for producing compressed air AK (see FIG. 2).

The detection section 10 is attached to a mount portion EPT of theexhaust pipe EP (a gas flow pipe) where a mount opening EPO is formed. Aportion of the detection section 10 (located on the right side (thedistal end side) of the mount portion EPT in FIG. 2) extends into theinterior of the exhaust pipe EP through the mount opening EPO and is tocome into contact with the exhaust gas EG (a gas under measurement).

Outside the exhaust pipe EP, the circuit section 201 is connected to thedetection section 10 through a cable 160 composed of a plurality ofwires. This circuit section 201 includes a circuit which drives thedetection section 10 and detects a signal current Is which will bedescribed later.

First, the electrical configuration of the circuit section 201 of thepresent system 1 will be described. The circuit section 201 has ameasurement control circuit 220 which includes a signal currentdetection circuit 230 and a heater energization circuit 226; an ionsource power supply circuit 210; and an auxiliary electrode power supplycircuit 240.

The ion source power supply circuit 210 has a first output terminal 211maintained at a first potential PV1 and a second output terminal 212maintained at a second potential PV2. Specifically, the second potentialPV2 is maintained at a positive high potential in relation to the firstpotential PV1. More specifically, a pulse voltage (1 to 2 kV0-p) whichis positive in relation to the first potential PV1 is output from thesecond output terminal 212. The pulse voltage is obtained throughhalf-wave rectification of a sinusoidal wave of about 100 kHz. Notably,the ion source power supply circuit 210 constitutes a constant-currentpower supply whose output current is feedback-controlled by amicroprocessor 202, which will be described later, such that the outputcurrent (rms or effective value) is maintained at a predeterminedcurrent value (for example, 5 μA). The microprocessor 202 can detect,through an unillustrated isolation amplifier circuit, the magnitude ofthe output current (discharge current Id to be described later) suppliedby the ion source power supply circuit 210.

Meanwhile, the auxiliary electrode power supply circuit 240 has anauxiliary first output terminal 241 which electrically communicates withthe first output terminal 211 and is maintained at the first potentialPV1, and an auxiliary second output terminal 242 which is maintained ata third potential PV3. Specifically, the third potential PV3 is set to apotential of, for example, DC 100 to 200 V which is a positive high DCpotential in relation to the first potential PV1 but is lower than thepeak potential (1 to 2 kV) of the second potential PV2.

Moreover, the signal current detection circuit 230, which partiallyconstitutes the measurement control circuit 220, has a signal inputterminal 231 connected to the first output terminal 211 of the ionsource power supply circuit 210, and a ground input terminal 232connected to a ground potential PVE. This signal current detectioncircuit 230 detects the signal current Is flowing between the signalinput terminal 231 and the ground input terminal 232.

Also, the heater energization circuit 226 is a circuit for energizing aheater 78 (which will be described later) through PWM control, tothereby cause the heater 78 to generate heat. The heater energizationcircuit 226 is connected to a first heater connection wiring line 169 aand a second heater connection wiring line 169 b of the cable 160.

In addition, the first output terminal 211 of the ion source powersupply circuit 210, the auxiliary first output terminal 241 of theauxiliary electrode power supply circuit 240, and the signal inputterminal 231 of the signal current detection circuit 230 are connectedto one another.

A primary-side core 271A of an isolation transformer 270 electricallycommunicates with the ground potential PVE, and a secondary-side core271B thereof electrically communicates with the first potential PV1 (thefirst output terminal 211 of the ion source power supply circuit 210).In the present embodiment, the measurement control circuit 220, the ionsource power supply circuit 210, and the auxiliary electrode powersupply circuit 240 are isolated from one anther by the isolationtransformer 270.

The measurement control circuit 220 includes a regulator power supplyPS. This regulator power supply PS is driven by an external battery BTthrough a power supply wiring line BC.

Also, the measurement control circuit 220 includes the microprocessor202, and can communicate, through a communication line CC, with acontrol unit ECU which controls the internal combustion engine. Thus,the measurement control circuit 220 can transmit to the control unitECU, for example, a reduced or converted value of the quantity ofparticulates S which corresponds to the magnitude of the signal currentIs detected by the signal current detection circuit 230.

The feed pump 300 takes in atmosphere (air) around the feed pump 300,and feeds clean, compressed air AK toward an ion source 11, which willbe described later, through an air feed pipe 310.

Next, the cable 160 will be described (see FIG. 2). A second potentialwiring line 161, an auxiliary potential wiring line 162, the firstheater connection wiring line 169 a, and the second heater connectionwiring line 169 b, which are formed of copper wire, and a hollow airpipe 163 formed of resin are disposed at the center of the cable 160.These wiring lines and pipe are circumferentially surrounded by a firstpotential wiring line 165 and a ground potential wiring line 167, eachformed of braided thin copper wires, with an unillustrated insulatorlayer disposed therebetween.

As described above, the circuit section 201 is connected to this cable160 (see FIG. 2). Specifically, the second output terminal 212 of theion source power supply circuit 210 is maintained at the secondpotential PV2, and is connected to the second potential wiring line 161so as to electrically communicate therewith. The auxiliary second outputterminal 242 of the auxiliary electrode power supply circuit 240 ismaintained at the third potential PV3, and is connected to the auxiliarypotential wiring line 162 so as to electrically communicate therewith.The first output terminal 211 of the ion source power supply circuit 210is maintained at the first potential PV1, and is connected to the firstpotential wiring line 165 so as to electrically communicate therewith.The ground input terminal 232 of the signal current detection circuit230 is connected, for electrical communication, to the ground potentialwiring line 167, whereby the ground input terminal 232 is maintained atthe ground potential PVE. The heater energization circuit 226 isconnected, for electrical communication, to the first heater connectionwiring line 169 a and the second heater connection wiring line 169 b.The air feed pipe 310 is connected to the air pipe 163 of the cable 160.

Next, the detection section 10 will be described (see FIG. 2). Asdescribed above, the detection section 10 is attached to the mountportion EPT of the exhaust pipe EP (gas flow pipe) of the engine ENG(internal combustion engine), the mount portion EPT having the mountopening EPO, and is to come into contact with the exhaust gas EG (gasunder measurement). From the viewpoint of the electrical functions ofthe detection section 10, the detection section 10 is mainly composed ofan ion source 11, a particulate electrification section 12, a firstconduction member 13, a needlelike electrode member 20, and an auxiliaryelectrode member 50.

The first conduction member 13, which is formed of metal and has acircular cylindrical shape, is connected to the first potential wiringline 165 at the distal end side of the cable 160, and electricallycommunicates with the first potential wiring line 165.

A distal end portion of the second potential wiring line 161 of thecable 160 is connected to the needlelike electrode member 20 inside thefirst conduction member 13. The needlelike electrode member 20 is formedof tungsten wire, and has a needlelike distal end portion 22 having asharp tip end. This needlelike distal end portion 22 serves as one ofthe two electrodes of the ion source 11, which will be described later.

Also, a distal end portion of the auxiliary potential wiring line 162 ofthe cable 160 is connected to an extending portion 51 of the auxiliaryelectrode member 50 inside the first conduction member 13. The auxiliaryelectrode member 50 is formed of stainless steel wire, a distal endportion of the auxiliary electrode member 50 is bent back to have aU-like shape, and the auxiliary electrode member 50 has an auxiliaryelectrode portion 53 at a distal end portion of the bent back portion.The auxiliary electrode portion 53 serves as an auxiliary electrodewhich will be described later. The extending portion 51 of the auxiliaryelectrode member 50 is covered by an auxiliary electrode insulating pipe79 with a heater (see FIG. 4). The heater-equipped auxiliary electrodeinsulating pipe 79 is composed of a cylindrical tubular, auxiliaryelectrode insulating pipe 77 formed of insulating ceramic such asalumina, the heater 78 formed on the surface of the auxiliary electrodeinsulating pipe 77 and united therewith, and an insulating ceramic layer76 covering them.

The heater-equipped auxiliary electrode insulating pipe 79 has twoheater terminals 78 a and 78 b of the heater 78 which are exposed to theoutside at the proximal end side (the lower side in FIG. 4) of theinsulating pipe 79. The heater 78 is formed of tungsten, and has heaterlead portions 78 r 1 and 78 r 2 extending from the heater terminals 78 aand 78 b toward the distal end side (the upper side in FIG. 4), and twoheating portions; i.e., a first heater portion 78 h 1 located at thedistal end and a second heater portion 78 h 2 located on the proximalend side in relation to the first heater portion 78 h 1. The firstheater portion 78 h 1 and the second heater portion 78 h 2 are connectedin parallel. The first heater portion 78 h 1 heats the vicinity of theauxiliary electrode portion 53 of the auxiliary electrode member 50,which serves as an auxiliary electrode. The second heater portion 78 h 2heats the vicinity of the ion source 11 (a nozzle member 31, which willbe described later, and the needlelike distal end portion 22 of theneedlelike electrode member 20). Namely, the heater 78 heats the ionsource 11 and the auxiliary electrode portion 53 (auxiliary electrode)of the auxiliary electrode member 50 by the first heater portion 78 h 1and the second heater portion 78 h 2, respectively.

The first conduction member 13 electrically communicates with the firstoutput terminal 211 of the ion source power supply circuit 210 throughthe first potential wiring line 165 of the cable 160, whereby the firstconduction member 13 is maintained at the first potential PV1. Also, thefirst conduction member 13 circumferentially surrounds a portion of theneedlelike electrode member 20 and a portion of the auxiliary electrodemember 50, which portions are located outside the exhaust pipe EP.

Further, the circumference of the first conduction member 13 issurrounded by a housing member 14 in such a manner that the firstconduction member 13 is insulated from the housing member 14. Thehousing member 14 is attached to the exhaust pipe EP and electricallycommunicates therewith. The housing member 14 is connected to the cable160 such that the housing member 14 electrically communicates with theground potential wiring line 167 of the cable 160 and is maintained atthe ground potential PVE.

The first heater connection wiring line 169 a and the second heaterconnection wiring line 169 b of the cable 160 are connected to heaterconnection terminals 170 a and 170 b, respectively, inside the firstconduction member 13. The heater connection terminals 170 a and 170 bare connected to the heater terminals 78 a and 78 b of the heater 78inside the first conduction member 13.

A distal end of the air pipe 163 of the cable 160 is opened inside thefirst conduction member 13. The compressed air AK supplied from the feedpump 300 through the air feed pipe 310 and the air pipe 163 of the cable160 is discharged from the air pipe 163, and is fed under pressure to adischarge space DS (which will be described later) located on the distalend side (right side in FIG. 2) of the air pipe 163.

The nozzle member 31 is fitted to a distal end portion (a right endportion in FIG. 2) of the first conduction member 13. A central portionof the nozzle member 31 is concaved toward the distal end side, and asmall through hole is formed at the center. The through hole serves as anozzle 31N. The nozzle member 31 electrically communicates with thefirst conduction member 13, and is maintained at the first potentialPV1.

As a result of the nozzle member 31 being fitted to the distal end ofthe first conduction member 13, the discharge space DS is formed insidethese members. In this discharge space DS, the projecting needlelikedistal end portion 22 of the needlelike electrode member 20 faces afacing surface 31T which is a surface of the nozzle member 31 on theproximal end side and which has a concave shape. Accordingly, when ahigh voltage is applied between the needlelike distal end portion 22 andthe nozzle member 31 (facing surface 31T), gaseous discharge occurs,whereby N₂, O₂, etc. in the atmosphere are ionized, whereby positiveions (e.g., N³⁺, O²⁺; hereinafter also referred to as “ions CP”) areproduced. The compressed air AK discharged from the air pipe 163 of thecable 160 is also supplied to the discharge space DS. Therefore, air ARoriginating from the compressed air AK is jetted at high speed from thenozzle 31N of the nozzle member 31 toward a mixing region MX (which willbe described later) located on the distal end side of the nozzle 31N,and the ions CP are also jetted toward the mixing region MX togetherwith the compressed air AK (air AR).

The particulate electrification section 12 is formed on the distal endside (on the right side in FIG. 2) of the nozzle member 31. An intakeport 331 and an exhaust port 43O, which are open toward the downstreamside of the exhaust pipe EP) are formed in the side wall of theparticulate electrification section 12. This particulate electrificationsection 12 communicates with the nozzle member 31 electrically as well.Therefore, the particulate electrification section 12 is maintained atthe first potential PV1.

The inner space of the particulate electrification section 12 isnarrowed by a collection electrode 42 which bulges inward, whereby aslit-shaped internal space is formed. As a result, on the proximal endside (on the left side in FIG. 2) of the collection electrode 42, acircular columnar space is formed between the nozzle member 31 and thecollection electrode 42.

Of the space inside the particulate electrification section 12, theabove-mentioned circular columnar space will be referred to as a“circular columnar mixing region MX1.” Also, the slit-shaped internalspace formed by the collection electrode 42 will be referred to as a“slit-shaped mixing region MX2” (see FIG. 3). The circular columnarmixing region MX1 and the slit-shaped mixing region MX2 will becollectively referred to as a “mixing region MX.” Further, a circularcolumnar space is also formed on the distal end side of the collectionelectrode 42, and serves as an exhaust passage EX which communicateswith the exhaust port 43O. In addition, on the proximal end side of thecollection electrode 42, there is formed an introduction passage HKwhich extends from the intake port 331 to the mixing region MX (thecircular columnar mixing region MX1).

Next, the electrical functions and operations of various sections of theparticulate detection system 1 of the present embodiment will bedescribed with reference to FIG. 3 in addition to FIG. 2. FIG. 3schematically shows the electrical function and operation of thedetection section 10 of the present system 1 in order to facilitate theunderstanding of the electrical function and operation.

The needlelike electrode member 20 is maintained at the second potentialPV2, which is a positive pulse voltage (1 to 2 kV0-p), which is obtainedthrough half-wave rectification of a sinusoidal wave of 100 kHz, inrelation to the first potential PV1 as described above. Meanwhile, theauxiliary electrode member 50 is maintained at the third potential PV3,which is a positive DC potential of 100 to 200 V in relation to thefirst potential PV1 as described above. Also, the first conductionmember 13, the nozzle member 31, and the particulate electrificationsection 12 are maintained at the first potential PV1. In addition, thehousing member 14 is maintained at the ground potential PVE, which isthe same as the potential of the ground input terminal 232 of the signalcurrent detection circuit 230 and the potential of the exhaust pipe EP.

Accordingly, as described above, positive needle corona PC, which iscorona around the needlelike distal end portion 22 serving as a positiveelectrode, is produced between the nozzle member 31 (the facing surface31T) maintained at the first potential PV1 and the needlelike distal endportion 22 maintained at the second potential PV2, which is a positivehigh potential in relation to the first potential PV1. As a result, N₂,O₂, etc. in the atmospheric air (air) therearound are ionized, wherebypositive ions CP are produced. Some produced ions CP pass through thenozzle 31N and are jetted toward the mixing region MX, together with theair AR originating from the compressed air AK supplied to the dischargespace DS. In the present embodiment, the needlelike distal end portion22 and the nozzle member 31 surrounding the discharge space DSconstitute the ion source 11 which generates ions CP by means of gaseousdischarge (corona discharge) between the needlelike distal end portion22 and the nozzle member 31.

When the air AR is jetted to the mixing region MX (the circular columnarmixing region MX1) through the nozzle 31N of the nozzle member 31, theair pressure in the circular columnar mixing region MX1 drops.Therefore, the exhaust gas EG is taken into the mixing region MX (thecircular columnar mixing region MX1, the slit-shaped mixing region MX2)from the intake port 331 through the introduction passage HK. Theintroduced exhaust gas EGI is mixed with the air AR, and is dischargedtogether with the air AR from the exhaust port 43O through the exhaustpassage EX.

At that time, if particulates S such as soot are contained in theexhaust gas EG, as shown in FIG. 3, the particulates S are alsointroduced into the mixing region MX. Incidentally, the jetted air ARincludes ions CP. Therefore, the ions CP adhere to the introducedparticulates S such as soot, and the particulates S become positivelyelectrified particulates SC. The positively electrified particulates SCare discharged, together with the introduced exhaust gas EGI and the airAR, from the exhaust port 43O through the mixing region MX and theexhaust passage EX.

Meanwhile, of the ions CP jetted to the mixing region MX, floating ionsCPF not having adhered to the particulates S receive a repulsive forcefrom the auxiliary electrode portion 53 of the auxiliary electrodemember 50, and adhere to portions of the particulate electrificationsection 12, which is maintained at the first potential PV1 and whichforms the collection electrode 42. As a result, the floating ions CPFare collected.

Next, there will be described the principle of detection of theparticulates S in the present system 1. As shown in FIG. 2, when gaseousdischarge occurs at the ion source 11, a discharge current Id issupplied to the needlelike distal end portion 22 from the second outputterminal 212 of the ion source power supply circuit 210. The greaterpart of the discharge current Id flows to the nozzle member 31 (receivedcurrent Ij). This received current Ij flows through the first conductionmember 13, and flows into first output terminal 211 of the ion sourcepower supply circuit 210.

The greater part of the ions CP produced at the ion source 11 and jettedtherefrom are collected by the collection electrode 42. A correctedcurrent Ih originating from the charge carried by the floating ions CPFcollected by the collection electrode 42 also flows into the firstoutput terminal 211 through the first conduction member 13, whichelectrically communicates with the collection electrode 42 (theparticulate electrification section 12). Namely, a received/collectedcurrent Ijh (=Ij+Ih) which is the sum of these currents flows throughthe first conduction member 13.

This received/collected current Ijh becomes slightly smaller inmagnitude than the discharge current Id. This is because some of theions CP produced at the ion source 11 adhere to the electrifiedparticulates SC released from the exhaust port 43O, whereby some of theions CP are released from the exhaust port 43O (the released ions willbe referred to as the “released ions CPH”). The received/collectedcurrent Ijh does not include a current component corresponding to thecharge of the released ions CPH. Notably, the exhaust pipe EP throughwhich the electrified particulates SC flow is maintained at the groundpotential PVE.

Incidentally, when viewed from the ion source power supply circuit 210,an imbalance occurs between the discharge current Id flowing out fromthe second output terminal 212 and the received/collected current Ijhflowing into the first output terminal 211. Therefore, a signal currentIs corresponding to the shortage (the difference=discharge current−thereceived/collected current) flows from the ground potential PVE to thefirst output terminal 211, whereby a balanced state is created.

In view of this, in the present system 1, the signal current detectioncircuit 230—which has the signal input terminal 231 communicating withthe first output terminal 211 and the ground input terminal 232communicating with the ground potential PVE and which detects the signalcurrent flowing between the signal input terminal 231 and the groundinput terminal 232—is provided so as to detect the signal current Iswhich flows from the ground potential PVE to the signal currentdetection circuit 230 through the housing member 14 and the groundpotential wiring line 167 of the cable 160, flows through the signalcurrent detection circuit 230, and then flows to the first outputterminal 211.

The magnitude of the signal current Is corresponding to the difference(the discharge current Id−the received/collected current Ijh) increasesand decreases in accordance with the quantity of charge of the releasedions CPH (ions adhering to the discharged, electrified particulates SC);accordingly, the quantity of particulates S in the introduced exhaustgas EGI; i.e., the quantity of particulates S contained in the exhaustgas EG flowing through the exhaust pipe EP. Accordingly, throughdetection of the signal current Is by the signal current detectioncircuit 230, there can be detected the quantity of particulates Scontained in the exhaust gas EG, which corresponds to the signal currentIs. Notably, in the present system 1, the converted value of thequantity of the particulates S is obtained from the detected signalcurrent Is through conversion performed through use of a predeterminedreference table.

Incidentally, as described above, the ion source power supply circuit210 constitutes a constant current power source, and the dischargecurrent Id (the gaseous discharge current in the present invention)supplied from the second output terminal 212 of the ion source powersupply circuit 210 to the needlelike distal end portion 22 isfeedback-controlled by the microprocessor 202 such that its rms value ismaintained at a predetermined current value (for example, 5 μA (=targetcurrent It)).

In order to accurately detect the quantity of particulates S through useof the signal current Is, the discharge current Id subjected to theconstant current control must be stable. However, in many cases, thedischarge current Id is unstable immediately after the operation of theion source 11 has been started by the ion source power supply circuit210. Also, when the insulation properties of the ion source 11 hasdeteriorated due to adhesion of condensed water or soot to thecircumference of the ion source 11, the discharge current Id repeatshunting or its initial value becomes excessively large as shown by acontinuous line and a broken line in a graph shown in FIG. 5. In such acase, the discharge current Id takes a great deal of time to converge tothe target current It. In such a state, the quantity of the ions CPgenerated by means of corona discharge varies. Therefore, if the signalcurrent Is is detected by the signal current detection circuit 230 inthe state in which the discharge current Id is unstable, difficulty isencountered in accurately detecting the quantity of the particulates S.

In view of the foregoing problem, in the system 1 of the presentembodiment, the detection of the particulates S through use of thesignal current Is detected by the signal current detection circuit 230is started after the discharge current Id (gaseous discharge current)supplied from the ion source power supply circuit 210 has converged to apredetermined allowable range IR after the operation of the ion source11 by the ion source power supply circuit 210 had been started.Specifically, the allowable range IR is set for the target current It(=5 μA) of the discharge current Id such that the lower limit Imin ofthe range becomes 4.5 μA and the upper limit Imax of the range becomes5.5 μA (see FIG. 5).

In addition, the present system 1 includes the heater 78 for heating theion source 11 and the auxiliary electrode portion 53 of the auxiliaryelectrode member 50, and the heater energization circuit 226 forenergizing the heater 78. Until the discharge current Id converges tothe allowable range IR after the ion source 11 has started itsoperation, the heater energization circuit 226 is caused to energize theheater 78 by means of PWM control, to thereby heat the ion source 11 andthe auxiliary electrode portion 53 of the auxiliary electrode member 50.

Further, in the present system 1, only when the discharge current Iddoes not converge to the allowable range IR within a predeterminedperiod of time (10 seconds in the present embodiment) after theoperation of the ion source 11 has been started, the heater energizationcircuit 226 is caused to start the energization of the heater 78.Accordingly, the energization of the heater 78 is not started in thecase where, at the start of the operation of the ion source 11, thedischarge current Id converges to the allowable range IR and thedetection of the quantity of the particulates S can be started. Byvirtue of this, it is possible to reduce the electric power consumed asa result of the energization of the heater 78.

Next, of operations of the present system 1, an operation of themicroprocessor 202 for executing a particulate detection routine will bedescribed with reference to the flowcharts of FIGS. 6 through 8.

First, in step S1 shown in FIG. 6, after performing necessary initialsetting, the microprocessor 202 starts the operation of the ion source11 by using the ion source power supply circuit 210. Notably, at thattime, the microprocessor 202 separately performs constant currentcontrol for maintaining the discharge current Id at a fixed level. As aresult, corona discharge is started.

In step S2 subsequent thereto, the microprocessor 202 executes aninitial convergence determination sub-routine shown in FIG. 7 so as todetermine whether or not the discharge current Id supplied from the ionsource power supply circuit 210 to the needlelike distal end portion 22of the ion source 11 converges to the allowable range IR (for example,Imin (=4.5 μA) to Imax (=5.5 μA)) before a predetermined period of time(10 sec in the present embodiment) elapses after the operation of theion source 11 has started in step S1.

Next, the initial convergence determination sub-routine of FIG. 7 willbe described.

In step S21 shown in FIG. 7, the microprocessor 202 sets the value of atime-up counter to 0. This time-up counter is used for measuring thepredetermined period of time (=10 sec.). In step S22 subsequent thereto,the microprocessor 202 sets the value of a convergence counter to 0.This convergence counter is used for determining whether or not thedischarge current Id has converged to the allowable range IR.

Further, in step S23 subsequent thereto, the microprocessor 202determines whether or not 10 msec has elapsed by using a timer whichclocks 10 msec. In the case where 10 msec has not yet elapsed (No), themicroprocessor 202 repeats this step S23. In the case where 10 msec haselapsed (Yes in step S23), the microprocessor 202 proceeds to step S24.As a result, every time 10 msec elapses, the processing of step S24 andsteps subsequent thereto is executed.

In step S24, the microprocessor 202 obtains the value of the dischargecurrent Id every time 10 msec elapses. In step S25 subsequent thereto,the microprocessor 202 increases the value of the time-up counter byone. Namely, the value of the time-up counter is increased by one everytime 10 msec elapses.

Further, in step S26 subsequent thereto, the microprocessor 202determines whether or not the value of the time-up counter becomes equalto or greater than 1000; namely, whether or not 10 sec (thepredetermined period of time) has elapsed after the start of thisinitial convergence determination sub-routine. In the case where 10 sechas not yet elapsed (No in step S26), the microprocessor 202 proceeds tostep S27.

In step S27, the microprocessor 202 determines whether or not thedischarge current Id obtained in step S24 falls within the allowablerange IR. In the case where the discharge current Id does not fallwithin the allowable range IR (No), the microprocessor 202 returns tostep S22 and resets the value of the convergence counter to 0.Subsequently, after waiting for elapse of 10 msec in step S23, themicroprocessor 202 again proceeds to step S24 so as to obtain thedischarge current Id. Meanwhile, in the case where the microprocessor202 determines in step S27 that the discharge current Id falls withinthe allowable range IR (Yes), the microprocessor 202 proceeds to stepS28 so as to increase the value of the convergence counter by one, andthen proceeds to step S29. In step S29, the microprocessor 202determines whether or not the value of the convergence counter is equalto or greater than 200. In the case where the value of the convergencecounter is not equal to or greater than 200 (No), the microprocessor 202returns to step S23 while maintaining the value of the convergencecounter. After that, the microprocessor 202 waits for elapse of 10 msecin this step S23, and again proceeds to step S24 so as to obtain thedischarge current Id.

In the case where the value of the convergence counter reaches 200during the repeated execution of steps S23 through S29; namely, in thecase where the value of the discharge current Id continuously fallswithin the allowable range IR for 2 sec, the result of the determinationin step S29 becomes “Yes,” and the microprocessor 202 proceeds to stepS2A. In step S2A, the microprocessor 202 determines that the dischargecurrent Id has converged to the allowable range IR, and sets aconvergence flag to 1. After that, the microprocessor 202 ends thisinitial convergence determination sub-routine.

Meanwhile, in the case where, during the repeated execution of steps S22through S27, the value of the time-up counter reaches 1000 before thevalue of the convergence counter reaches 200, the result of thedetermination in step S26 becomes “Yes,” and the microprocessor 202proceeds to step S2B. In step S2B, the microprocessor 202 determinesthat the discharge current Id failed to converge to the allowable rangeIR within the predetermined period of time (=10 sec), and sets aconvergence flag to 0. After that, the microprocessor 202 ends thisinitial convergence determination sub-routine.

Upon completion of the initial convergence determination sub-routine ofFIG. 7, the microprocessor 202 proceeds to step S3 of FIG. 6.

In step S3, the microprocessor 202 determines whether or not theconvergence flag is 1; namely, whether or not the discharge current Idhas converged to the allowable range IR. In the case where theconvergence flag is 1 (the discharge current Id has converged to theallowable range IR) (Yes); namely, the detection of the quantity of theparticulates S can be started from the outset of the operation of theion source 11, the microprocessor 202 proceeds to step S8 so as to startthe detection of the quantity of the particulates S through use of thesignal current Is.

Meanwhile, in the case where the microprocessor 202 determines in stepS3 that the convergence flag is 0 (No); namely, the discharge current Idfailed to converge to the allowable range IR, the microprocessor 202proceeds to step S4.

In step S4, the microprocessor 202 causes the heater energizationcircuit 226 to start the energization of the heater 78 by means of PWMcontrol, to thereby heat the ion source 11 and the auxiliary electrodeportion 53 of the auxiliary electrode member 50.

In step S5 subsequent thereto, the microprocessor 202 executes aconvergence determination sub-routine shown in FIG. 8 so as to determinewhether or not the discharge current Id has converged to the allowablerange IR.

Next, the convergence determination sub-routine of FIG. 8 will bedescribed.

In step S51 shown in FIG. 8, the microprocessor 202 sets the value of atime-up counter to 0. This time-up counter is used for interrupting thedetermination of this convergence determination sub-routine after elapseof 3 min. In step S52 subsequent thereto, the microprocessor 202 setsthe value of a convergence counter to 0. This convergence counter isused for determining whether or not the discharge current Id hasconverged to the allowable range IR.

Further, in step S53 subsequent thereto, the microprocessor 202determines whether or not 10 msec has elapsed by using a timer whichclocks 10 msec. In the case where 10 msec has not yet elapsed (No), themicroprocessor 202 repeats this step S53. In the case where 10 msec haselapsed (Yes in step S53), the microprocessor 202 proceeds to step S54.As a result, every time 10 msec elapses, the processing of step S54 andsteps subsequent thereto is executed.

In step S54, the microprocessor 202 obtains the value of the dischargecurrent Id every time 10 msec elapses. In step S55 subsequent thereto,the microprocessor 202 increases the value of the time-up counter byone. Namely, the value of the time-up counter is increased by one everytime 10 msec elapses.

Further, in step S56 subsequent thereto, the microprocessor 202determines whether or not the value of the time-up counter becomes equalto or greater than 18000; namely, whether or not 3 min (180 sec), whichis the period for interrupting the determination, has elapsed after thestart of this convergence determination sub-routine. In the case where 3min has not yet elapsed (No in step S56), the microprocessor 202proceeds to step S57.

In step S57, the microprocessor 202 determines whether or not thedischarge current Id obtained in step S54 falls within the allowablerange IR.

In the case where the discharge current Id does not fall within theallowable range IR (No), the microprocessor 202 returns to step S52 andresets the value of the convergence counter to 0. Subsequently, afterwaiting for elapse of 10 msec in step S53, the microprocessor 202 againproceeds to step S54 so as to obtain the discharge current Id.

Meanwhile, in the case where the microprocessor 202 determines in stepS57 that the discharge current Id falls within the allowable range IR(Yes), the microprocessor 202 proceeds to step S58 so as to increase thevalue of the convergence counter by one, and then proceeds to step S59.In step S59, the microprocessor 202 determines whether or not the valueof the convergence counter is equal to or greater than 200.

In the case where the value of the convergence counter is not equal toor greater than 200 (No), the microprocessor 202 returns to step S53while maintaining the value of the convergence counter. After that, themicroprocessor 202 waits for elapse of 10 msec in this step S53, andagain proceeds to step S54 so as to obtain the discharge current Id.

In the case where the value of the convergence counter reaches 200during the repeated execution of steps S53 through S59; namely, in thecase where the value of the discharge current Id continuously fallswithin the allowable range IR for 2 sec, the result of the determinationin step S59 becomes “Yes,” and the microprocessor 202 proceeds to stepS5A. In step S5A, the microprocessor 202 determines that the dischargecurrent Id has converged to the allowable range IR, and sets theconvergence flag to 1. After that, the microprocessor 202 ends thisconvergence determination sub-routine.

Meanwhile, in the case where, during the repeated execution of steps S52through S57, the value of the time-up counter reaches 18000 before thevalue of the convergence counter reaches 200, the result of thedetermination in step S56 becomes “Yes,” and the microprocessor 202proceeds to step S5B. In step S5B, the microprocessor 202 sets theconvergence flag to 0. After that, the microprocessor 202 ends thisconvergence determination sub-routine. In this case, the dischargecurrent Id failed to converge to the allowable range IR within 3 min.

Upon completion of the convergence determination sub-routine of FIG. 8,the microprocessor 202 proceeds to step S6 of FIG. 6.

In step S6, the microprocessor 202 determines whether or not theconvergence flag is 1; namely, whether or not the discharge current Idhas converged to the allowable range IR. In the case where theconvergence flag is 1 (the discharge current Id has converged to theallowable range IR) (Yes), the microprocessor 202 proceeds to step S7.In step S7, the microprocessor 202 causes the heater energizationcircuit 226 to stop the energization of the heater 78. After that, themicroprocessor 202 proceeds to step SS8, and starts the detection of thequantity of the particulates S through use of the signal current Is.

Meanwhile, in the case where the microprocessor 202 determines in stepS6 that the convergence flag is 0 (No); namely, in the case where it isdetermined by the convergence determination sub-routine that thedischarge current Id failed to converge to the allowable range IR within3 min, the microprocessor 202 proceeds to step S9. In step S9, themicroprocessor 202 causes the heater energization circuit 226 to stopthe energization of the heater 78. In step S10 subsequent thereto, themicroprocessor 202 performs error processing necessary for interruptingthe processing. In this case, the microprocessor 202 ends theparticulate detection routine without performing the particulatedetection.

As described above, in the system 1 of the present embodiment, after thedischarge current Id (gaseous discharge current) supplied to the ionsource 11 has converged to the predetermined allowable range IR (e.g.,Imin (=4.5 μA) to Imax (=5.5 μA)) (Yes in step S3, S6) after theoperation of the ion source 11 had been started (step S1), the detectionof the quantity of the particulates S through use of the signal currentIs is started (step S8).

As a result, the detection of the quantity of the particulates S can bestarted in a state in which the discharge current Id is stable.Therefore, the quantity of the generated ions CP becomes stable, and thequantity of the particulates S can be detected accurately.

Further, the system 1 of the present embodiment includes the heater 78for heating the ion source 11, and the heater energization circuit 226for energizing the heater 78. After the ion source 11 has started itsoperation, the heater energization circuit 226 is caused to energize theheater 78 to thereby heat the ion source 11 until the discharge currentId converges to the allowable range IR (steps S2 to S7).

By virtue of this, it is possible to remove foreign substances, such aswater droplets (e.g., condensed water) and soot, which adhere to thecircumference of the ion source 11, to thereby recover the insulationproperties of the ion source 11. Thus, the discharge current Id can bequickly converged to the allowable range IR, whereby the time requiredto start the detection of the quantity of the particulates S can beshortened.

Further, in the system 1 of the present embodiment, when the dischargecurrent Id fails to converge to the allowable range IR within apredetermined period of time after the ion source 11 has started itsoperation, the heater energization circuit 226 is caused to start theenergization of the heater 78 (steps S2 to S4).

Accordingly, in the system 1, the energization of the heater 78 is notperformed, in the case where, at the start of the operation of the ionsource 11, the discharge current Id converges to the allowable range IRand the detection of the quantity of the particulates S can be started.Thus, the electric power consumed as a result of the energization of theheater 78 can be reduced.

Further, in the system 1 of the present embodiment, the auxiliaryelectrode member 50 has the auxiliary electrode portion 53 which isprovided at a distal end portion thereof and which serves as anauxiliary electrode, and the heater 78 heats the auxiliary electrodeportion 53 of the auxiliary electrode member 50 in addition to the ionsource 11. By virtue of this, it is possible to recover the insulationproperties of the auxiliary electrode portion 53 having deteriorated asa result of adhesion of foreign substances, such as condensed water andsoot, to the auxiliary electrode portion 53 serving as an auxiliaryelectrode. Therefore, the quantity of the particulates S can be detectedmore properly.

In the present embodiment, the signal current Is corresponds to thesignal corresponding to the quantity of electrified particulates SC inthe present embodiment. Also, the signal current detection circuit 230of the measurement control circuit 220 which detects the signal currentIs corresponds to the detection circuit of the present invention, andthe signal current detection circuit 230 and the microprocessor 202corresponds to the control section of the present invention. Also, theion source power supply circuit 210 corresponds to the drive circuit andthe ion source drive circuit of the present invention, and the auxiliaryelectrode power supply circuit 240 corresponds to the drive circuit andthe auxiliary electrode drive circuit of the present invention.

Further, the microprocessor 202 which executes steps S2 to S7corresponds to the heater energization control means of the presentinvention, and the microprocessor 202 which executes steps S2 to S4corresponds to the heater energization start means of the presentinvention.

Also, the microprocessor 202 which executes steps S2 to S3 and steps S5to S6 corresponds to the current convergence determination means of thepresent invention, and the microprocessor 202 which executes step S8corresponds to the detection start means of the present invention.

In the above, the present invention has been described on the basis ofthe system 1 of the embodiment. However, the present invention is notlimited to the above-described embodiment, and may be modified freelywithout departing from the scope of the invention.

For example, in the embodiment, the heater-equipped auxiliary electrodeinsulating pipe 79 in which the heater 78 is integrally formed on thesurface of the auxiliary electrode insulating pipe 77 covering thecircumference of the extending portion 51 of the auxiliary electrodemember 50 is provided, and the ion source 11 and the auxiliary electrodeportion 53 of the auxiliary electrode member 50 are heated by the heater78 of the heater-equipped auxiliary electrode insulating pipe 79.However, the form of the heater is not limited thereto, and a heater forheating the ion source 11 only may be provided. Also, there may beprovided two heaters for separately heating the ion source 11 and theauxiliary electrode portion 53 of the auxiliary electrode member 50.

Also, the embodiment may be modified to wait until the discharge currentId converges to the allowable range IR without heating the ion source11, etc. by the heater. Further, the embodiment may be modified toexecute the first initial convergence determination (step S2) afterstarting the energization of the heater 78 immediately after the startof the operation of the ion source 11.

Also, in the embodiment, in step S7, the heater energization circuit 226is caused to stop the energization of the heater 78. However, theembodiment may be modified to decrease the duty ratio of PWM control instep S7 so as to supply a small amount of electric power to the heater78, to thereby continue slight heat generation.

Also, in the embodiment, the signal current Is is converted to thequantity of the particulates S through use of a predetermined referencetable. However, the signal current Is may be converted to the quantityof the particulates S through use of a predetermined conversionequation. Also, the magnitude of the signal current Is itself may beused as a physical quantity corresponding to the quantity of theparticulates S. Further, in the embodiment, the needlelike electrodemember 20 is disposed in the discharge space DS. However, the embodimentmay be modified in such a manner that the needlelike distal end portion22 of the needlelike electrode member 20 is disposed to face the mixingarea MX, gaseous discharge is produced between the needlelike distal endportion 22 and the inner surface of the particulate electrificationsection 12 forming the mixing area MX.

DESCRIPTION OF SYMBOLS

-   AM: car (vehicle)-   ENG: engine (internal combustion engine)-   EP: exhaust pipe (gas flow pipe)-   EG: exhaust gas-   EGI: introduced exhaust gas-   S: particulates-   SC: electrified particulates-   CP: ions-   CPF: floating ions-   CPH: released ions-   Id: discharge current (gaseous discharge current)-   Is: signal current-   1: particulate detection system-   10: detection section-   11: ion source-   12: particulate electrification section-   20: needlelike electrode member-   22: needlelike distal end portion (of the needlelike electrode    member) (ion source)-   31: nozzle member (ion source)-   42: collection electrode-   50: auxiliary electrode member-   53: auxiliary electrode portion (of the auxiliary electrode member)    (auxiliary electrode)-   78: heater-   202: microprocessor (control section)-   201: circuit section-   210: ion source power supply circuit (drive circuit, ion source    drive circuit)-   220: measurement control circuit-   226: heater energization circuit-   230: signal current detection circuit (control section, detection    circuit)-   240: auxiliary electrode power supply circuit (drive circuit,    auxiliary electrode drive circuit)-   It: target current-   IR: allowable range-   S2 to S4: heater energization start means-   S2 to S7: heater energization control means-   S2 to S3, S5 to S6: current convergence determination means-   S8: detection start means

The invention claimed is:
 1. A particulate detection system fordetecting the quantity of particulates contained in a gas undermeasurement flowing through a gas flow pipe, comprising: a detectionsection attached to the gas flow pipe; a drive circuit for driving thedetection section; and a control section for controlling the drivecircuit and detecting the quantity of the particulates, wherein thedetection section includes an ion source for generating ions by means ofgaseous discharge, and a particulate electrification section for mixinga portion of the gas under measurement with the ions to thereby produceelectrified particulates which originate from the particulates withinthe gas under measurement and which carry the ions adhering thereto; thedrive circuit includes an ion source drive circuit for performingconstant current control such that the gaseous discharge currentsupplied to the ion source becomes equal to a predetermined targetcurrent; and the control section includes a detection circuit fordetecting a signal corresponding to the quantity of the electrifiedparticulates, current convergence determination means for determiningwhether or not the gaseous discharge current supplied from the ionsource drive circuit has converged to a predetermined allowable rangeincluding the target current after operation of the ion source by theion source drive circuit had been started, and detection start means forstarting detection of the quantity of the particulates through use ofthe signal after the gaseous discharge current has converged to theallowable range.
 2. A particulate detection system according to claim 1,wherein the detection section includes a heater for heating the ionsource; the drive circuit includes a heater energization circuit forenergizing the heater; and the control section includes heaterenergization control means for causing the heater energization circuitto energize the heater until the gaseous discharge current converges tothe allowable range.
 3. A particulate detection system according toclaim 2, wherein the heater energization control means includes heaterenergization start means for causing the heater energization circuit tostart the energization of the heater when the gaseous discharge currentdoes not converge to the allowable range within a predetermined periodof time after the operation of the ion source has been started.
 4. Aparticulate detection system according to claim 2, wherein the detectionsection includes a collection electrode for collecting floating ionswhich are a portion of the ions and which failed to adhere to theparticulates when the ions was mixed with the gas under measurement, andan auxiliary electrode for assisting the collection of the floating ionsby the collection electrode; the drive circuit includes an auxiliaryelectrode drive circuit for driving the auxiliary electrode; and theheater is configured to heat the auxiliary electrode in addition to theion source.